Droplet splash control for extreme ultraviolet photolithography

ABSTRACT

A photolithography system utilizes tin droplets to generate extreme ultraviolet radiation for photolithography. The photolithography system irradiates the droplets with a laser. The droplets become energized and emit extreme ultraviolet radiation. A collector reflects the extreme ultraviolet radiation toward a photolithography target. The photolithography system reduces splashback of the tin droplets onto the receiver by generating a net electric charge within the droplets using a charge electrode and decelerating the droplets by applying an electric field with a counter electrode.

BACKGROUND Technical Field

The present disclosure relates to the field of photolithography. Thepresent disclosure relates more particularly to extreme ultravioletphotolithography.

Description of the Related Art

There has been a continuous demand for increasing computing power inelectronic devices including smart phones, tablets, desktop computers,laptop computers and many other kinds of electronic devices. Integratedcircuits provide the computing power for these electronic devices. Oneway to increase computing power in integrated circuits is to increasethe number of transistors and other integrated circuit features that canbe included for a given area of semiconductor substrate.

The features on an integrated circuit die are produced, in part, withthe aid of photolithography. Traditional photolithography techniquesinclude generating a mask outlining the shape of features to be formedon an integrated circuit die. They photolithography light sourceirradiates the integrated circuit die through the mask. The size of thefeatures that can be produced via photolithography of the integratedcircuit die is limited, in part, on the lower end, by the wavelength oflight produced by the photolithography light source. Smaller wavelengthsof light can produce smaller feature sizes.

Extreme ultraviolet light is used to produce particularly small featuresdue to the relatively short wavelength of extreme ultraviolet light. Forexample, extreme ultraviolet light is typically produced by irradiatingdroplets of selected materials with a laser beam. The energy from thelaser causes the droplets to enter a plasma state. In the plasma state,the droplets emit extreme ultraviolet light. The extreme ultravioletlight travels toward a collector with an elliptical or parabolicsurface. The collector reflects the extreme ultraviolet light onto thephotolithography target.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram of a photolithography system, according to oneembodiment.

FIG. 2 is a block diagram of a photolithography system, according to oneembodiment.

FIG. 3 is a functional flow diagram of a process for reducing splashbackin a photolithography system.

FIG. 4 is a method for reducing splashback in a photolithographyprocess, according to an embodiment.

FIG. 5 is a method for reducing splashback in a photolithographyprocess, according to an embodiment.

DETAILED DESCRIPTION

In the following description, many thicknesses and materials aredescribed for various layers and structures within an integrated circuitdie. Specific dimensions and materials are given by way of example forvarious embodiments. Those of skill in the art will recognize, in lightof the present disclosure, that other dimensions and materials can beused in many cases without departing from the scope of the presentdisclosure.

The following disclosure provides many different embodiments, orexamples, for implementing different features of the described subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present description. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various embodiments of thedisclosure. However, one skilled in the art will understand that thedisclosure may be practiced without these specific details. In otherinstances, well-known structures associated with electronic componentsand fabrication techniques have not been described in detail to avoidunnecessarily obscuring the descriptions of the embodiments of thepresent disclosure.

Unless the context requires otherwise, throughout the specification andclaims that follow, the word “comprise” and variations thereof, such as“comprises” and “comprising,” are to be construed in an open, inclusivesense, that is, as “including, but not limited to.”

The use of ordinals such as first, second and third does not necessarilyimply a ranked sense of order, but rather may only distinguish betweenmultiple instances of an act or structure.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. Thus, the appearances of the phrases “in one embodiment” or“in an embodiment” in various places throughout this specification arenot necessarily all referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be combined inany suitable manner in one or more embodiments.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contentclearly dictates otherwise. It should also be noted that the term “or”is generally employed in its sense including “and/or” unless the contentclearly dictates otherwise.

FIG. 1 is a block diagram of a photolithography system 100 in accordancewith one embodiment. The photolithography system 100 includes a laser102, a photolithography target 104, a collector 106, a droplet generator108, a droplet receiver 110, a charge electrode 112, and a controlsystem 114. The droplet receiver 110 includes a droplet pool 116, acounter electrode 118, and a droplet sensor 120. The components of thephotolithography system 100 cooperate to reduce splashback from dropletsonto the collector 106.

The droplet generator 108 generates and outputs a stream of droplets.The droplets can include, in one example, liquid (melted) tin. Othermaterials can be used for the droplets without departing from the scopeof the present disclosure. The droplets move at a high rate of speedtoward the droplet receiver 110. The photolithography system 100utilizes the droplets to generate extreme ultraviolet light forphotolithography processes. Extreme ultraviolet light typicallycorresponds to light with wavelengths between 5 nm and 125 nm.

The laser 102 outputs a laser beam. The laser beam is focused on a pointthrough which the droplets pass on their way from the droplet generator108 to the droplet receiver 110. In particular, the laser 102 outputslaser pulses. Each laser pulse is received by a droplet. When thedroplet receives the laser pulse, the energy from the laser pulsegenerates a high-energy plasma from the droplet. The high-energy plasmaoutputs extreme ultraviolet radiation.

In one embodiment, the radiation output by the plasma scatters randomlyin many directions. The photolithography system 100 utilizes thecollector 106 to collect the scattered extreme ultraviolet radiationfrom the plasma droplets and reflect the extreme ultraviolet radiationtoward a photolithography target 104, or toward equipment that willguide the extreme ultraviolet radiation to the photolithography target104.

In one embodiment, the collector 106 includes an aperture. The laserpulses from the laser 102 pass through the aperture toward the stream ofdroplets. This enables the collector 106 to be positioned between thelaser 102 and the photolithography target 104.

After the droplets have been irradiated by the laser 102, the dropletscontinue with a trajectory toward the droplet receiver 110. The dropletreceiver 110 receives the droplets in a droplet pool 116. The dropletpool 116 collects the droplets. The droplets can be drained from thedroplet pool 116 and reused or disposed of.

It is possible that some splashback from the droplets may fall upon thecollector 106. This is because the droplets travel with high velocity.The high velocity of the droplets can result in splashing. Thissplashing, or splashback, can travel back toward the collector 106.

Droplet splashback on the collector 106 can have adverse effects on thephotolithography system 100. For example, the splashback can result inan uneven surface of the collector 106. If the surface of the collector106 is bumpy or otherwise uneven, the collector 106 may not reflect theextreme ultraviolet radiation toward the photolithography target 104. Itis also possible that the accumulation of splashback on the reflector106 will cause the collector 106 to become substantially nonreflective.In either of these cases, the photolithography system 100 may noteffectively irradiate the photolithography target 104 with extremeultraviolet radiation.

The photolithography system 100 utilizes the charge electrode 112 andthe counter electrode 118 to reduce splashback on the collector 106.Voltages are applied to the charge electrode 112 and the counterelectrode 118. As will be described in more detail below, the voltagesapplied to the charge electrode 112 and the counter electrode 118 canassist in reducing splashback of the droplets onto the collector 106.

In one embodiment, a voltage is applied to the charge electrode 112. Thecharge electrode 112 is positioned so that droplets output by thedroplet generator 108 pass adjacent to the charge electrode 112 as theytravel toward the droplet receiver 110. Because a high voltage isapplied to the charge electrode 112, as the droplets pass adjacent tothe charge electrode 112, free charges in the droplets are attracted tothe charge electrode 112. The result is that the droplets gain a netelectric charge. In particular, the droplets have a net electric chargewith a polarity opposite to the polarity of the voltage applied to thecharge electrode 112.

In one embodiment, imparting a charge to the droplets is facilitated bythe fact that the droplets are in a plasma state. In the plasma state,valence electrons are freed from their atoms, enabling the electrons tomove freely. The individual conductivity of charged particles in theplasma state enables the free charges to be drawn to the chargeelectrode 112.

In one embodiment, the charge electrode 112 has a positive voltage. Freeelectrons in the droplets are drawn to the charge electrode 112. Theresult is that the droplets have a net positive charge due to thesmaller number of electrons than protons in the droplets. Thus, thecharge electrode 112 generates positively charged droplets, in oneexample.

In one embodiment, the counter electrode 118 carries a voltage or a netcharge with a same polarity as the voltage or net charge on the chargeelectrode 112. Accordingly, as the charged droplets approach the counterelectrode 118, the charged droplets experience a repulsiveelectromagnetic force. The repulsive electromagnetic force reduces thevelocity of the droplets. Because the velocity of the droplets isreduced, the droplets produce little or no splashback.

In one embodiment, a higher voltage is applied to the counter electrode118 than to the charge electrode 112. The reason for this is that afterthe charge electrode 112 has charged the droplets, the droplets may beaccelerated by the electrostatic force between the charge electrode 112and the charged droplets as the charged droplets move beyond the chargeelectrode 112 toward the droplet receiver 110. Accordingly, the counterelectrode 118 may have a voltage or a net charge higher than the voltageor net charge on the charge electrode 112. In this way, the counterelectrode 118 can decelerate the charged droplets to a greater degreethan the charge electrode 112 can accelerate the charged droplets.

In one embodiment, the photolithography system 100 includes a dropletsensor 120. The droplet sensor 120 senses the speed of the droplets asthey are received into the droplet pool 116. This measurement of thespeed of the droplets can be utilized to calibrate or fine-tune thevoltages applied to the charge electrode 112 and the counter electrode118. The droplet sensor 120 generates sensor signals indicative of thespeed of the droplets as they pass through the droplet receiver 110.

The control system 114 is coupled to the droplet sensor 120. The controlsystem 114 receives the sensor signals from the droplet sensor 120. Thecontrol system 114 processes the sensor signals. The control system 114can determine whether the droplets are sufficiently decelerated based onthe sensor signals from the droplet sensor 120.

The control system 114 is coupled to the charge electrode 112 and thecounter electrode 118. The control system 114 can control voltagesapplied to the charge electrode 112 and to the counter electrode 118. Inparticular, the control system 114 can control the voltages applied tothe charge electrode 112 and the counter electrode 118 based on thesensor signals received from the droplet sensor 120. If the sensorsignals indicate that the speed of the droplets is still too high as thedroplets are received into the droplet pool 116, then the control system114 can adjust the voltages applied to the charge electrode 112 and/orto the counter electrode 118.

The primary force that slows the droplets is the repulsiveelectromagnetic force between the counter electrode 118 and thedroplets. The electromagnetic force results from the voltage applied tothe counter electrode 118 and the net charge carried by the droplets.The electromagnetic force is repulsive because the net charge carried bythe droplets is the same polarity as the voltage applied to the counterelectrode 118. As the charged droplets approach the counter electrode118, the repulsive force slows down the droplets.

The primary factors that determine the strength of the electromagneticforce are the magnitude of the net charge carried by the droplets andthe magnitude of the voltage applied to the counter electrode 118.Increasing either of these quantities results in a greater repulsiveelectromagnetic force. The net charge carried by the droplets isdetermined, primarily, by the magnitude of the voltage applied to thecharge electrode 112. A higher voltage applied to the charge electrode112 will result in a higher net charge on the droplets. Accordingly,increasing either of the voltages applied to the charge electrode 112and the counter electrode 118 will result in greater deceleration of thedroplets. Thus, the control system 114 can adjust the speed of thedroplets by increasing a voltage on either or both of the chargeelectrode 112 and the counter electrode 118.

In one embodiment, the counter electrode 118 includes multiple counterelectrodes 118 positioned in the droplet receiver 110. The multiplecounter electrodes are arranged along a path of travel of the dropletsthrough the droplet receiver. The counter electrodes 118 closer to thecharging electrode carry a lower voltage than the counter electrodesfurther from the charge electrode 112, though all counter electrodes 118carry the same polarity of voltage as the charge electrode 112. Theincreasing voltage of the counter electrodes 118 ensures that thedroplets will continue to decelerate as the travel through the dropletreceiver 110.

In one embodiment, the droplet receiver 110 has a conductive shell orbody. The conductive shell 118 of the droplet receiver can function asthe counter electrode 118. A voltage can be applied to the conductiveshell to decelerate the droplets as they pass through the dropletreceiver. The conductive shell can carry a charge gradient such that themagnitude of a charge or voltage carried by the conductive shellincreases with increasing distance from the charge electrode 112.Accordingly, the droplet receiver 110 can include a dynamically chargedconductive shell that acts as the counter electrode.

In one embodiment, the control system 114 adjusts the speed of thedroplets by increasing a voltage of the counter electrode 118 relativeto the charge electrode 112. After the droplets pass through the chargeelectrode 112, the droplets will experience an acceleration due to therepulsive electromagnetic force between the charge electrode 112 and thenow charged droplets. Accordingly, an effective way to decrease thespeed of the droplets is to increase the voltage applied to the counterelectrode 118 relative to the charge electrode 112. This will ensurethat the deceleration experienced by the droplets as they approachcounter electrode is greater than the acceleration experienced by thedroplets as the travel away from the charge electrode 112. Nevertheless,the control system 114 can reduce the speed of the droplets by adjustingthe voltages applied to either or both of the charge electrode 112 andthe counter electrode 118.

In one example, the control system 114 can increase the voltage appliedto the charge electrode 112 in order to impart a greater net charge tothe droplets. The control system 114 can apply a corresponding voltageincrease to the counter electrode 118. In some embodiments, the controlsystem can apply a greater increase in voltage to the counter electrode118 than to the charge electrode 112. In some embodiments, the controlsystem 114 can apply the same voltage increase to both the chargeelectrode 112 and the counter electrode 118. In some embodiments, thecontrol system 114 can adjust the voltage of only one of the counterelectrode 118 and the charge electrode 112. In some embodiments, thecontrol system 114 can increase the voltage on one of the chargeelectrode 112 of the counter electrode 118 and decrease the voltage onthe other of the charge electrode 112 and the counter electrode 118.

Another possible way to reduce splashback is to increase the temperatureof the droplets. The droplets may effectively become stickier withincreasing temperature. The sticker droplets may not splashback as muchupon impacting a back area of the droplet receiver 110.

As used herein, the voltage applied to the charge electrode is termed afirst voltage. The voltage applied to the counter electrode 118 istermed a second voltage. The first and second voltages may or may not bea same value.

In one embodiment, the control system 114 includes or is coupled to apower source. The power source supplies voltages to the charge electrode112 and to the counter electrode 118. The control system 114 can controlthe voltages supplied by the power source to the charge electrode 112and the counter electrode 118.

In one embodiment, the control system 114 is coupled to the dropletgenerator 108. The control system 114 can apply a voltage to the dropletgenerator 108 or to a portion of the droplet generator 108. For example,in one embodiment, the control system 114 can apply a ground voltage tothe droplet generator 108 in order to generate an initial voltage dropbetween the droplets and the charge electrode 112.

The control system 114, the droplet sensor 120, the charge electrode112, and the counter electrode 118, act as a feedback loop. The feedbackloop controls the speed of the droplets. More particularly, the feedbackloop measures and adjusts the speed of the droplets to reduce oreliminate splashback of the droplets onto the collector 106.

In one embodiment, the control system 114 utilizes machine learning toaccurately adjust the speed of the droplets. Accordingly, the controlsystem 114 can include a machine learning model that can be trained toadjust the voltages applied to the charge electrode 112 and the counterelectrode 118 responsive to the sensor signals provided by the dropletsensor 120 in order to accurately achieve a desired speed for thedroplets.

In one embodiment, the machine learning model includes a decision treemodel. The decision tree model receives input data regarding thedroplets, the charge electrode, and the counter electrode. In oneexample, the input data can include the mass of the droplets, thetemperature of the droplets, the current value of the speed of thedroplets, a previous value of the speed of the droplets, a current valueof the voltage on the charge electrode 112, a current value of thevoltage on the counter electrode 118, a previous value of the voltage onthe charge electrode 112, and a previous value of the voltage on thecounter electrode 118. The previous value of the speed of the dropletscan correspond to the speed of the droplets prior to a most recentvoltage adjustment made by the control system 114. The previous value ofthe voltage on the charge electrode 112 can correspond to the value ofthe charge on the charge electrode 112 prior to a most recent voltageadjustment made by the control system 113. The previous value of thevoltage on the counter electrode 118 can correspond to the value of thecharge on the counter electrode prior to a most recent voltageadjustment made by the control system 114. The input data can beprovided to the decision tree model as a vector or series of vectorshaving data values representative of the values described above.

In one embodiment, the decision tree model includes a plurality ofdecision nodes. Each decision node includes a decision rule. Thedecision rule at a decision node applies to one or more of the inputvalues. For example, one or more decision nodes may apply a ruleregarding the current speed of the droplets. One or more decision nodesmay apply a rule regarding the previous speed of the droplets, and soforth for the various types of the input data. In one example, a firstdecision node applies a rule that if the current speed is within aselected range the follow path a first path to a next decision node, ifnot, follow path a second path to a next decision node. Next nodes mayapply rules regarding the previous speed of the droplets or the currentmass of the droplets, for example.

Each decision node can include two or more decision paths that each leadto another decision node. A final layer of decision nodes determines theaction, if any, to be taken by the control system. Accordingly, thevalues of the various input data fields determine the path that will betraveled through the decision tree until the input data arrives at oneof a plurality of possible final decisions or classifications. Thevarious possible final decisions or classifications can includeincreasing the voltage on the charge electrode 112, increasing thevoltage on the counter electrode 118, decreasing the voltage on thecharge electrode 112, decreasing the voltage on the counter electrode118, increasing the voltage on both the charge electrode 112 and thecounter electrode 118, decreasing the voltage on both the chargeelectrode 112 and the counter electrode 118, increasing the voltage onthe charge electrode 112 and decreasing the voltage on the counterelectrode 118, decreasing the voltage on the charge electrode 112 andincreasing the voltage on the counter electrode 118, increasing thetemperature of the droplets, decreasing the temperature of the droplets,and making no adjustment at all. The final decision or classificationdetermines the action that will be taken by the control system 114. Whenthe final decision or classification includes an adjustment to thevoltage on one or both of the charge electrode 112 and the counterelectrode 118, the final decision or classification corresponds tovoltage adjustment data. The control system 114 adjusts the voltages inaccordance with the voltage adjustment data.

In one embodiment, the decision tree model is trained with anunsupervised machine learning process. During the unsupervised machinelearning process the decision tree model is given a directive to adjustparameters of the photolithography system 100 in order to adjust ormaintain the speed of the droplets within a selected speed range orbelow a selected speed threshold. During the unsupervised machinelearning process, the machine learning model sets up a number ofdecision nodes and decision rules. Droplets are passed through thecharge electrode 112 into the droplet receiver 110. The speed of thedroplets is measured, and the various other input data are provided tothe decision tree. The various voltages are adjusted in accordance withthe final decision and the process repeats. Throughout the machinelearning process, the decision nodes, pathways, decision rules and finaldecisions are adjusted in iterations until the control system 114 isable to reliably achieve and maintain the selected speed for thedroplets. In this way, the decision tree learns an algorithm that canmaintain the speed of the droplets in the desired range.

There is a potentially very large number of combinations of decisionnodes and rules, i.e., algorithms, that can achieve the desired dropletspeed results. The final selected algorithm is determined by the machinelearning process and the initial parameters provided to the decisiontree model by experts. The initial parameters can include initialdecision nodes and decision rules. In an alternative embodiment, themachine learning process can include a supervised machine learningprocess utilizing training sets from past measurements of droplet speed,electrode voltages, and adjustments.

In one embodiment, the machine learning model includes a neural networkbased algorithm. The neural network based algorithm can include any oneof a plurality of neural network including, but not limited to,recurrent neural networks, convolutional neural networks, deepconvolutional neural networks, and feed forward neural networks. Theinput to the neural network can include the same types of input datadescribed above in relation to the decision tree network. The neuralnetwork includes a series of neural layers with weighted neurons thatcollectively determine one of a plurality of possible actions to betaken by the control system 114 based on the input data. The machinelearning process can include supervised machine learning processes, deeplearning processes, or unsupervised machine learning processes. Othertypes of machine learning models than those described above can beutilized for controlling the speed of droplets without departing fromthe scope of the present disclosure.

In one embodiment, the control system 114 does not utilize a machinelearning model. Instead, the control system 114 can execute a simplealgorithm that determines what adjustments should be made based on thetypes of input data described above in relation to the machine learningmodels. The algorithm can be defined by human experts and programmedinto the control system 114. Those of skill in the art will recognize,in light of the present disclosure, that many types of algorithms can beutilized to achieve a selected droplet speed, without departing from thescope of the present disclosure.

FIG. 2 is an illustration of a photolithography system 200, according toan embodiment. The photolithography system 100 includes a laser 102, aphotolithography target 104, a collector 106, a droplet generator 108, adroplet receiver 110, a charge electrode 112, and a control system 114.

The droplet generator 108 generates and outputs a stream of droplets124. The droplets can include, as described previously, tin. Thedroplets 124 move at a high rate of speed toward the droplet receiver110.

The laser 102 is positioned behind a collector 106. The laser 102outputs pulses of laser light 132. The pulses of laser light 132 arefocused on a point through which the droplets pass on their way from thedroplet generator 108 to the droplet receiver 110. Each pulse of laserlight 132 is received by a droplet 124. When the droplet 124 receivesthe pulse of laser light 132, the energy from the laser pulse generatesa high-energy plasma from the droplet 124. The high-energy plasmaoutputs extreme ultraviolet radiation.

In one embodiment, the laser 102 is a carbon dioxide (CO2) laser. TheCO2 laser emits radiation or laser light 132 with a wavelength centeredaround 9.4 μm or 10.6 μm. The laser 102 can include lasers other thancarbon dioxide lasers and can output radiation with other wavelengthsthan those described above without departing from the scope of thepresent disclosure.

In one embodiment the droplet generator 108 generates between 40,000 and60,000 droplets per second. The droplets 124 have an initial velocity ofbetween 70 m/s and 90 m/s. The droplets have a diameter between 10 μmand 200 μm. The droplet generator 108 can generate different numbers ofdroplets per second than described above without departing from thescope of the present disclosure. The droplet generator 108 can alsogenerate droplets having different initial velocities and diameters thanthose described above without departing from the scope of the presentdisclosure.

In one embodiment, the laser 102 irradiates each droplet 124 with twopulses. A first pulse causes the droplet 124 to flatten into a disk likeshape. The second pulse causes the droplet 124 to form a hightemperature plasma. The second pulse is significantly more powerful thanthe first pulse. The laser 102 and the droplet generator 108 arecalibrated so that the laser 102 emits pairs of pulses such that eachdroplet 124 is irradiated with a pair of pulses. For example, if thedroplet generator 108 outputs 50,000 droplets per second, the laser 102will output 50,000 pairs of pulses per second. The laser 102 canirradiate droplets 124 in a manner other than described above withoutdeparting from the scope of the present disclosure. For example, thelaser 102 may irradiate each droplet 124 with a single pulse or withmore pulses than two.

In one embodiment, the droplets 124 are tin. When the tin droplets 124are converted to a plasma, the tin droplets 124 output extremeultraviolet radiation 134 with a wavelength centered between 10 nm and15 nm. More particularly, in one embodiment the tin plasma shines with acharacteristic wavelength of 13.5 nm. These wavelengths correspond toextreme ultraviolet radiation. Materials other than tin can be used forthe droplets 124 without departing from the scope of the presentdisclosure. Such other materials may generate extreme ultravioletradiation with wavelengths other than those described above withoutdeparting from the scope of the present disclosure.

In one embodiment, the radiation 134 output by the droplets scattersrandomly in many directions. The photolithography system 100 utilizesthe collector 106 to collect the scattered extreme ultraviolet radiation134 from the plasma and output the extreme ultraviolet radiation towarda photolithography target 104.

In one embodiment, the collector 106 is a parabolic or ellipticalmirror. The scattered radiation 134 is collected and reflected by theparabolic or elliptical mirror with a trajectory toward aphotolithography target 104.

In one embodiment, the collector 106 includes an aperture 135. Thepulses of laser light 132 pass from the laser 102 through the aperture135 toward the stream of droplets 124. This enables the collector 106 tobe positioned between the laser 102 and the photolithography target 104.

After the droplets 124 have been irradiated by the laser 102, thedroplets 124 continue with a trajectory toward the droplet receiver 110.In particular, the droplets enter an opening 126 in the droplet receiver110 and travel through an interior passage 128 toward a droplet pool 116at a back end of the inner passage 128. The droplet pool 116 collectsthe droplets 124. The droplet receiver 110 can further include a drainport (not shown) that drains the droplet pool 116. The droplets 124 canbe reused or disposed of.

In order to reduce or eliminate splashback from the droplets 124, thephotolithography system 100 includes a ring shaped, annular, or toroidalcharge electrode 112. The charge electrode 112 is positioned so that thedroplets 124 pass through a center of the ring shaped charge electrode112. The charge electrode 112 is positioned downstream from the locationat which the droplets 124 are irradiated, and upstream from the dropletreceiver 110.

The charge electrode 112 is coupled to the control system 114. Thecontrol system 114 applies, or causes to be applied, a voltage to thecharge electrode 112. In one embodiment, the voltage applied to thecharge electrode is between 100 V 40,000 V, though other voltages can beapplied to the charge electrode 112 without departing from the scope ofthe present disclosure. The voltage can be selected to be sufficient todraw free electrons from the droplets 124 to the charge electrode 112.Higher voltages on the charge electrode 112 will draw larger numbers ofelectrons from each droplet 124. The larger the number of electronsdrawn from a droplet 124, the greater the net positive charge on thedroplet. The greater the net positive charge, the greater the reductionin speed that can be achieved via the counter electrode 118. The chargeelectrode 112 can also carry a net positive charge such that theelectrode 112 generates an electric field in the vicinity of the chargeelectrode 112.

The droplet receiver 130 includes an outer casing 110. The outer casing130 encloses the interior passage 128. The counter electrode 118 ispositioned in the casing 130 adjacent to a backend of the interiorpassageway 128. The counter electrode 118 includes a cup shape such thatthe counter electrode 118 surrounds a backend of the interior passageway128. The counter electrode 118 can include other shapes and positionswithout departing from the scope of the present disclosure.

The counter electrode 118 is coupled to the control system 114. Thecontrol system 114 applies, or causes to be applied, a voltage to thecounter electrode 118. In one embodiment, the counter electrode 118carries a voltage or a net charge with a same polarity as the voltage ornet charge on the charge electrode 112. Accordingly, as the chargeddroplets approach the counter electrode 118, the charged dropletsexperience a repulsive electromagnetic force. The repulsiveelectromagnetic force reduces the velocity of the droplets. Because thevelocity of the droplets is reduced, the droplets produce little or nosplashback.

In one embodiment, a higher voltage is applied to the counter electrode118 than to the charge electrode 112. The reason for this is that afterthe charge electrode 112 has charged the droplets, the droplets may beaccelerated by the electromagnetic force between the charge electrode112 and the charged droplets as the charged droplets move beyond thecharge electrode 112 toward the droplet receiver 110. Accordingly, thecounter electrode 118 may have a voltage or net charge that is higherthan the voltage or net charge on the charge electrode 112. In this way,the counter electrode 118 can decelerate the charged droplets to agreater degree than the charge electrode 112 can accelerate the chargeddroplets.

In one embodiment, the voltage applied to the counter electrode 118 isbetween 1000 V and 60,000 V, though other voltages can be applied to thecharge electrode 112 without departing from the scope of the presentdisclosure.

The droplet sensor 120 is positioned within or adjacent to a backend ofthe interior passage 128. In the example of FIG. 2, the droplet sensor120 includes a laser sensor that senses the velocity or speed of thedroplets 124 as the droplets approach the droplet pool 116. The lasersensor can include laser and a sensor receiver. The laser outputs alaser beam toward the sensor receiver. As a droplet passes through thelaser beam, the laser beam is interrupted such that the sensor receiverdoes not receive the laser beam for a duration of time corresponding tothe time required for the droplet to entirely pass through the laserbeam. If the size of the droplet 124 is known or estimated, then thespeed or velocity of the droplet 124 can be calculated or estimatedbased on the duration of the interruption.

The sensor receiver can pass sensor signals to the control system 114indicative of the speed or velocity of the droplets 124. The sensorsignals can include digital or analog signals. The sensor signals caninclude data explicitly indicating the speed of the droplets.Alternatively, the sensor signals can include analog waveformsindicative of the interruptions, and thereby indicative of the speed.Other schemes for sensor signals can be used without departing from thescope of the present disclosure. Other types of droplet sensors 120 canbe used without departing from the scope of the present disclosure.

The control system 114 receives the sensor signals from the dropletsensor 120. The control system 114 processes the sensor signals. Thecontrol system 114 can determine whether the droplets are sufficientlydecelerated based on the sensor signals from the droplet sensor 120.

The control system 114 can include one or more controllers orprocessors. The control system 114 can include one or more computermemories that can store instructions and data. The controllers orprocessors can execute the instructions and process the data.

The control system 114 can control the voltages applied to the chargeelectrode 112 and the counter electrode 118 based on the sensor signalsreceived from the droplet sensor 120. If the sensor signals indicatethat the speed of the droplets is too high as the droplets are receivedinto the droplet pool 116, then the control system 114 can adjust thevoltages applied to the charge electrode 112 and/or to the counterelectrode 118.

In one example, the control system 114 can take steps to reduce thespeed of the droplets by increasing the voltages applied to the chargeelectrode 112 and the counter electrode 118. In one example, the controlsystem 114 can increase the voltage applied to the charge electrode 112in order to impart a greater net charge to the droplets. The controlsystem 114 can apply a corresponding voltage increase to the counterelectrode 118. In some circumstances, the control system can apply agreater increase in voltage to the counter electrode 118 than to thecharge electrode 112. In some circumstances, the control system 114 canapply the same voltage increase to both the charge electrode 112 and thecounter electrode 118. In some circumstances, the control system 114 canadjust the voltage of only one of the counter electrode 118 and thecharge electrode 112. In some circumstances, the control system 114 canincrease the voltage on one of the charge electrode 112 or the counterelectrode 118. The control system 114 can decrease the voltage on theother of the charge electrode 112 and the counter electrode 118. Thecontrol system 114 of FIG. 2 can utilize the same types of actions asthose described above in relation to FIG. 1.

In one embodiment, the control system 114 includes or is coupled to apower source. The power source supplies voltages to the charge electrode112 and to the counter electrode 118. The control system 114 can controlthe voltages supplied by the power source to the charge electrode 112and the counter electrode 118.

In one embodiment, the control system 114 is coupled to the dropletgenerator 108. The control system 114 can apply a voltage to the dropletgenerator 108 or to a portion of the droplet generator 108. For example,in one embodiment, the control system 114 can apply a ground voltage tothe droplet generator 108 in order to generate an initial voltage dropbetween the droplets and the charge electrode 112. Alternatively, thecontrol system 114 utilize another ground voltage or reference voltage.

In one embodiment, the control system 114 can utilize machine learningmodels or other types of algorithms to achieve the desired speed of thedroplets including, but not limited to, the machine learning models andalgorithms described above in relation to FIG. 1.

FIG. 3 is a functional flow diagram of a process for 300 for controllingsplashback of droplets in a photolithography process, according to anembodiment. The process 300 is performed by a control system 114including a machine learning model 150.

At 302, the machine learning model 150 receives sensor data from adroplet sensor, utilizing any of the components and processes describedin relation to FIGS. 1 and 2. The sensor data can include the sensorsignals provided by the droplet sensor. Alternatively, the sensor datacan include data derived from the sensor signals provided by the dropletsensor. Accordingly, receiving sensor data from the droplet sensor caninclude receiving sensor data derived from sensor signals received fromthe droplet sensor.

At block 304, the machine learning model analyzes the sensor data,utilizing any of the components or processes described in relation toFIGS. 1 and 2. The machine learning model 150 can include a model thathas been trained with a machine learning process, in the case of asupervised machine learning model. The supervised machine learningprocess can train the machine learning model to generate voltageadjustment data that will result in a desired speed of the droplets inorder to reduce splashback. The supervised machine learning process cantrain the machine learning model to generate the voltage adjustment databased on the sensor data. Accordingly, the supervised machine learningprocess can utilize the training set data that includes measuredsplashback and sensor data. Alternatively, the machine learning model150 can include an unsupervised machine learning model. Other machinelearning models and processes can be utilized for the machine learningmodel 150 without departing from the scope of the present disclosure.The machine learning model can include, but is not limited to, the typesof machine learning models described above in relation to FIG. 1.

At 306, the machine learning model generates voltage adjustment databased on the sensor data. The voltage adjustment data indicatesadjustments that should be made to either or both of the voltagesapplied to a charge electrode and a counter electrode. The voltageadjustment data can also indicate that no adjustment should be made tothe voltages.

At 308, if the voltage adjustment data indicates that voltage adjustmentshould be made, the process proceeds to 310. At 310, the control system114 adjusts the voltages applied to the charge electrode and/or thecounter electrode based on the voltage adjustment data. From 310, theprocess returns to 302.

At 308, if the voltage adjustment data indicates that the voltagesshould not be adjusted, the process returns to 302.

By utilizing the machine learning model 150, the control system 114 canadjust the voltages applied to the counter electrode and the chargeelectrode in iterations. The process repeats itself until the speed ofthe droplets is in an acceptable range. Thereafter, the process repeatsitself to maintain the speed of the droplets in the acceptable range. Inthis way, the control system 114 can operate to reduce splashback of thedroplets onto the collector.

FIG. 4 is a method 400 for reducing splashback of droplets in aphotolithography system, according to an embodiment. At 402, the method400 includes outputting a stream of droplets from a droplet generator.One example of a droplet generator is the droplet generator 108 of FIG.1 or FIG. 2. At 404, the method 400 includes irradiating the droplets ofa laser. One example of a laser is a laser 102 of FIG. 1 or FIG. 2. At406, the method 400 includes receiving the droplets in a dropletreceiver. One example of a droplet receiver is the droplet receiver 110of FIG. 1 or FIG. 2. At 408, the method 400 includes generating a netcharge in the droplets upstream from the droplet receiver. At 410, themethod 400 includes reducing a speed of the droplets within the dropletreceiver by generating an electric field within the droplet receiver.

FIG. 5 is a method 500 for reducing splashback of droplets in aphotolithography system, according to an embodiment. At 502, the method500 includes outputting a plurality of droplets with a dropletgenerator. One example of a droplet generator is the droplet generator108 of FIG. 1 or FIG. 2. At 504, the method 500 includes generatingextreme ultraviolet radiation by irradiating the droplets with a laser.One example of a laser is the laser 102 of FIG. 1 or FIG. 2. At 506, themethod 500 includes inducing a net electric charge in the droplets byapplying a first voltage to a charge electrode positioned adjacent to apath of the droplets. One example of the charge electrode is the chargeelectrode 112 of FIG. 1 or FIG. 2. At 508, the method 500 includesreducing a speed of the droplets by applying a second voltage to acounter electrode positioned downstream from the charge electrode. Oneexample of the counter electrode is the counter electrode 118 of FIG. 1or FIG. 2.

In one embodiment, a photolithography system includes a dropletgenerator configured to output a stream of droplets and a dropletreceiver positioned to receive the droplets. The system includes a laserconfigured to irradiate the droplets and a collector configured toreceive extreme ultraviolet radiation from the droplets and to reflectthe extreme ultraviolet radiation for use in photolithography. Thesystem includes a charge electrode positioned between the dropletgenerator and the droplet receiver. The system includes a counterelectrode positioned downstream from the charge electrode with respectto a direction of travel of the droplets. The system includes a controlsystem configured to apply a first voltage to the charge electrode and asecond voltage to the counter electrode. The first voltage is selectedto impart a net electric charge to the droplets as the droplets passadjacent to the charge electrode. The second voltage is selected todecelerate the droplets as the droplets pass adjacent to the counterelectrode.

In one embodiment, a method includes outputting a stream of dropletsfrom a droplet generator, irradiating the droplets with a laser, andreceiving the droplets in a droplet receiver. The method includesgenerating a net charge in the droplets upstream from the dropletreceiver and reducing a speed of the droplets within the dropletreceiver by generating an electric field within the droplet receiver.

In one embodiment, a method includes outputting a plurality of dropletswith a droplet generator and generating extreme ultraviolet radiation byirradiating the droplets with a laser. The method includes inducing anet electric charge in the droplets by applying a first voltage to acharge electrode positioned adjacent to a path of the droplets andreducing a speed of the droplets by applying a second voltage to acounter electrode positioned downstream from the charge electrode.

The various embodiments described above can be combined to providefurther embodiments. Aspects of the embodiments can be modified, ifnecessary, to employ concepts of the various patents, applications andpublications to provide yet further embodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. A photolithography system, comprising: a droplet generator configuredto output a stream of droplets; a droplet receiver positioned to receivethe droplets; a laser configured to irradiate the droplets; a collectorconfigured to receive extreme ultraviolet radiation from the dropletsand to reflect the extreme ultraviolet radiation for use inphotolithography; a charge electrode positioned between the dropletgenerator and the droplet receiver; a counter electrode positioneddownstream from the charge electrode with respect to a direction oftravel of the droplets; and a control system configured to apply a firstvoltage to the charge electrode and a second voltage to the counterelectrode, the first voltage being selected to impart a net electriccharge to the droplets as the droplets pass adjacent to the chargeelectrode, the second voltage being selected to decelerate the dropletsas the droplets pass adjacent to the counter electrode.
 2. Thephotolithography system of claim 1, wherein the first voltage and thesecond voltage have a same polarity.
 3. The photolithography system ofclaim 2, wherein the second voltage has a greater magnitude than thefirst voltage.
 4. The photolithography system of claim 1, furthercomprising a droplet sensor configured to generate sensor signalsindicative of a speed of the droplets within the droplet receiver. 5.The photolithography system of claim 4, wherein the control system isconfigured to receive the sensor signals and to adjust one or both ofthe first and second voltages responsive to the sensor signals.
 6. Thephotolithography system of claim 5, wherein the control system includesa machine learning model that analyzes the sensor signals and outputsvoltage adjustment data based on the sensor signals.
 7. Thephotolithography system of claim 6, wherein the control system adjustsone or both of the first and second voltages based on the voltageadjustment data.
 8. The photolithography system of claim 7, wherein themachine learning model includes a neural network.
 9. Thephotolithography system of claim 7, wherein the machine learning modelincludes a decision tree model.
 10. A method comprising: outputting astream of droplets from a droplet generator; irradiating the dropletswith a laser; receiving the droplets in a droplet receiver; generating anet charge in the droplets upstream from the droplet receiver; andreducing a speed of the droplets within the droplet receiver bygenerating an electric field within the droplet receiver.
 11. The methodof claim 10, wherein generating the net charge in the droplets includesapplying a first voltage to a charge electrode positioned upstream fromthe droplet receiver.
 12. The method of claim 11, wherein generating theelectric field includes applying a second voltage to a counterelectrode.
 13. The method of claim 12, further comprising: generatingsensor signals indicative of a speed of the droplets within the dropletreceiver; passing the sensor signals to a control system; and adjusting,with the control system, one or both of the first and second voltagebased on the sensor signals.
 14. The method of claim 13, furthercomprising: passing the sensor signals, or data derived from the sensorsignals to a machine learning model; analyzing the sensor signals or thedata derived from the sensor signals with the machine learning model;generating, with the machine learning model, voltage adjustment databased, at least in part, on the sensor signals; and adjusting one orboth of the first and second voltages based on the sensor signals. 15.The method of claim 14, further comprising generating the voltageadjustment data based, at least in part, on a mass of the droplets. 16.The method of claim 14, further comprising generating the voltageadjustment data based, at least in part, on a previous speed of thedroplets.
 17. The method of claim 12, further comprising generatingextreme ultraviolet radiation from the droplets by irradiating thedroplets with the laser.
 18. The method of claim 17, further comprisingperforming photolithography with the extreme ultraviolet radiation. 19.A method, comprising: outputting a plurality of droplets with a dropletgenerator; generating extreme ultraviolet radiation by irradiating thedroplets with a laser; inducing a net electric charge in the droplets byapplying a first voltage to a charge electrode positioned adjacent to apath of the droplets; and reducing a speed of the droplets by applying asecond voltage to a counter electrode positioned downstream from thecharge electrode.
 20. The method of claim 19, further comprising:generating sensor signals indicative of a speed of the droplets withinthe droplet receiver; passing the sensor signals to a control system;and adjusting, with the control system, one or both of the first andsecond voltages based on the sensor signals.